In a combination vehicle of a type of a semitrailer or a full trailer, in general, a trailer towed by a tractor is pivotably coupled with a pin or a coupler provided on the rear part of the tractor. In such a type of combination vehicle, it is known that a coupling angle (hitch angle) between a tractor and a trailer increases rapidly in the quick braking, quick steering or reverse travelling of the vehicle, so that a “Jackknife phenomenon”, namely, a behavioral unstable condition in which the vehicle is bent in the V shape at the pin or the coupler, is liable to occur. Then, so far, there are proposed various solutions for preventing the jackknife phenomenon in a combination vehicle through controlling braking-driving forces on wheels of a tractor and/or a trailer using a braking control device, such as ABS, etc. For example, patent document 1 has proposed a coupler between a tractor and a trailer, which coupler incorporates therein a braking control mechanism which generates a braking force difference between right and left wheels in the trailer to generate a yaw moment in the direction of the decreasing of a hitch angle. Also, patent document 2 discloses controlling independently braking forces of the right and left wheels of a vehicle with a braking control device of an electronic controlled type when a risk of a jackknife phenomenon is detected. Furthermore, in patent documents 3-5, it is proposed to brake a trailer at an early stage for preventing a jackknife phenomenon owing to the pushing of a tractor by the trailer in the start of a yaw moment control of the tractor, in the start of an automatic deceleration control or in the braking during slope descending. And, in patent document 6, it is proposed to prevent a jackknife phenomenon by providing wheels of a trailer with a driving means, such as a motor etc., and adjusting the driving forces of the right and left wheels of the trailer.                Patent document 1: Japanese Laid-open Patent Publication No. 51-108419        Patent document 2: Japanese Laid-open Patent Publication No. 10-1037        Patent document 3: Japanese Laid-open Patent Publication No. 2000-43695        Patent document 4: Japanese Laid-open Patent Publication No. 2000-43705        Patent document 5: Japanese Laid-open Patent Publication No. 2001-278019        Patent documents 6: Japanese Laid-open Patent Publication No 10-157652        